Even if you find a working logic board, you should make sure that any connector/plug pins ( eg which connect to the motherboard (eg: Screen connector, HDD connector, or battery connector) have not shortened out. If these pins are damaged, then you might have to consider replacing these connectors as well. Also, check whether the battery is in not affected by your water damage
Ok, if its a liquid damage, there is a high possibility that plug which connects the battery and the motherboard damaged. The pins of the plug may have been damaged due to electric shorts when the pins were exposed to the liquid. You might want to buy the plug and perhaps replace it to the one soldered to the motherboard. It might be the case
Yes It is possible to replace your SSD with a higher capacity one. To my understanding, replacing your SSD with a higher capacity one is possible as long as the voltage ratings are suitable. Try some brands like Samsung or crucial !